Thermoplastic polyester resins are produced by subjecting a dicarboxylic acid component (including an ester-forming derivative thereof; the same shall apply hereinafter) and a dihydroxy compound component (including an ester-forming derivative thereof; the same shall apply hereinafter); a hydroxy carboxylic acid compound component (including an ester-forming derivative thereof; the same shall apply hereinafter); or all of these three components to an esterification or transesterification reaction to form a polyester precursor and then polycondensing the precursor. In the polycondensation step, the polyester precursor is heated by stirring under reduced pressure. Excessive monomer(s) and by-products are removed from the reaction system. So that the polycondensation reaction may proceed. Therefore, it is quite important to remove these substances quite efficiently and thus reduce the necessary reaction time to produce the polymer product in an economically advantageous manner or to obtain a stable high-quality polymer. Particularly the manner of stirring is important, since it exerts a significant influence on the efficiency of degasification and removal of these substances.
Known stirring processes in the polycondensation step include, for example, a process wherein the stirring is conducted at a constant rotational speed and a process wherein the rotational speed is lowered stepwise as the viscosity of the reaction mixture is increased with the progress of polycondensation. However, in the polycondensation step, the viscosity of the reaction mixture is continuously increased as the reaction proceeds and the viscosity becomes extremely high particularly immediately before the completion of the reaction. Such a change in the viscosity makes efficient stirring difficult.
When the rated output of the stirring motor is constant in the former process, the following phenomenon occurs: when the rotational speed is fixed on a high level suitable for a low-viscosity region in the initial stage of the reaction as shown in FIG. 4, it cannot adapt itself to the stirring in a high-viscosity region in the latter half of the reaction and, therefore, the stirring motor is stopped by the overload. When, on the contrary, the rotational speed is fixed on a low level suitable for the high viscosity in the latter half of the reaction, the stirring in the initial stage of the reaction is quite inefficient as shown in FIG. 3.
In the latter process wherein the rotational speed is lowered stepwise (usually in 2 to 5 steps) as the viscosity of the reaction mixture is increased as shown in FIG. 2, efficient stirring is possible. However, immediately after the rotational speed is lowered, a superfluous allowance over the rated output occurs and the efficiency of degasification and removal of the excess monomers, etc. will be insufficient.
Thus, in the known stirring processes, the stirring efficiency is poor and, therefore, degasification of the excess monomer(s), etc. is insufficient. Thus, problems occur such as reduced polymerization rates prolonged, reaction times reduced degrees of polymerization and inconsistent polymer quality.
The present invention relates to a process for producing a polyester comprising as the main structural unit(s) any of the following components (1) to (3):
(1) a dicarboxylic acid residue and a dihydroxy compound residue, PA1 (2) a hydroxy carboxylic acid compound residue, and PA1 (3) a dicarboxylic acid residue, a dihydroxy compound residue and a hydroxy carboxylic acid compound residue,
by a polycondensation reaction in a molten state under stirring, characterized in that the polycondensation is conducted while the stirring rate is reduced continuously in a stepless manner corresponding to an increase in the melt viscosity as the reaction proceeds so that the stirring load is substantially constant after the viscosity of the reaction mixture has reached an arbitrary value between 20 and 800 P until the completion of the reaction.